During ringing, it is necessary that telephone systems should be capable of detecting when the ringing telephone is taken off-hook, i.e. when it is answered, and this detection must take place as rapidly and as reliably as possible.
Generally speaking a looped subscriber line must be detected regardless of whether the subscriber is making a call or receiving a call. When the subscriber is making a call, loop detection is not much of a problem, however the same is not true for a called subscriber, i.e. when a subscriber unhooks a ringing telephone. In current telephone systems, a direct voltage is superposed on the ringing waveform, and the loop is detected by virtue of a direct current (DC) component which flows when the telephone is taken off-hook. Such detection becomes very difficult when the alternating current (AC) in the line due to the AC component of the ringing current is considerably greater than the DC component to be detected. Further, the wide variety of line impedances and ringing impedances makes it necessary to design worst case telephone systems which are generally not optimum in performance for the majority of telephone installations.
A subscriber is connected to a telephone exchange by a line of resistance RL and the ringer is connected to the terminals of the line. From an electrical point of view the ringer can be approximated by a resistance Rs connected in series with a capacitance Cs. Likewise across the terminals of the line, and thus in parallel with the ringer, there is the remainder of the subscriber station which from an electrical point of view is equivalent to a resistance Rt connected in series with a switch K, where the switch represents the gravity-operated hook-switch of the telephone set. The line is supplied with power by a DC generator which delivers a direct voltage of 48 volts in most cases. A ringing source constituted by an AC generator supplies the line with an r.m.s. voltage of 80 volts for example and is connected in series with the DC generator. In the on-hook mode, i.e. for a called subscriber, only the AC generator causes a current to flow and the value of the current is determined by the line resistance RL and by the ringer impedance, i.e. the resistance Rs and the capacitance Cs in series. In off-hook mode, i.e. once the subscriber has taken his telephone off hook, the DC generator delivers direct current at a value which is determined by the resistances RL and Rt, and the AC generator delivers a current which is determined by the resistance of the line RL in series with an impedance constituted by the resistance Rt in parallel with the impedance of the ringer, and thus in parallel on the series connection of the resistance Rs and the capacitance Cs. The line current in off-hook mode thus comprises a DC component and an AC component. Except in special cases, the ratio of these components is generally different from the ratio of the voltages supplied by the two generators.
Conventional systems for detecting a loop during ringing consist in detecting the DC component of the line current.
One circuit of this type comprises a resistance bridge having two resistances in series with each wire of the line, each of which constitutes one arm of the bridge. The other two arms of the bridge are each constituted by two resistances in series and the mid-point of each of these two arms is connected to a differential amplifier which delivers an output voltage proportional to the instantaneous transverse current in the line. It is not easy to detect a loop directly from this signal by comparing it with a detection threshold since the amplitude of the AC component in the signal is much larger than the threshold. Thus, before the telephone is taken off-hook the line current has no DC component but only an AC component generated by the AC generator such that the signal delivered by the differential amplifier only includes an AC component and goes larger than the threshold during each positive half cycle of the AC component. After the telephone is taken off-hook, the line current includes a DC component together with the AC component so long as ringing continues, i.e. so long as the AC generator applies ringing current to the line. The signal delivered by the differential amplifier thus includes a DC component and an AC component whose amplitude is such that it drops below the threshold during the negative half cycles thereof. This requires the signal to be filtered in such a manner as to attentuate the AC component and this must be done well enough to ensure that its amplitude always remains below the detection threshold before the telephone goes off-hook and that its amplitude after the telephone goes off-hook is never so great as to bring the signal back below the threshold during negative half cycles.
The signal from the amplifier is thus filtered by a filter comprising a resistance R and a capacitance C before being applied to the input of a comparator whose other input receives the detection threshold. The RC time constant of the filter is designed for the worst case, i.e. for the case of a short line on which the AC ringing current is of maximum amplitude since the line resistance is low. In this case the observed time constant is about 80 milliseconds and, in practice, in order to retain a safety margin, a value of 100 milliseconds is adopted. Under such conditions the worst case for detecting a loop during ringing becomes a long line of high resistance since the DC component in the line is small, being about 20 milliamps. The AC ringing current in the line is smaller than in the case of a short line and the signal at the output of the amplifier as applied to the filter includes a smaller AC component than it does in the short line case. At the output from the filter the AC component of the signal applied to the comparator is small relative to the DC component and it may be ignored. Under such conditions using a detection threshold corresponding to a line current of 10 milliamps, the loop detection time Td is about 70 milliseconds.
In electronic junctors, it is advantageous for reasons of dissipation and technology to reduce the supply voltages as much as possible. If the electronic junctor generates the ringing current, the DC voltage is reduced since it is not possible to reduce the AC ringing voltage very much and in some types of electronic junctor the AC generator operates at 60 volts r.m.s. for the ringing current while a DC generator operates at 12 volts for powering the subscriber's telephone. Under such conditions, detecting a loop during ringing becomes even more difficult. The filter time constant increases from about 80 to about 250 milliseconds, and the detection time increases considerably and approaches the standard maximum laid down by Telephone Administrations which is about 250 milliseconds.
Generally speaking the detection system must be designed for the worst case in which the amplitude of the AC component in the line current is greatest. In this case an electronic junctor operating with a 12 volt DC generator suffers from increased difficulty because of the reduced ratio between the DC and the AC components.
Account must also be taken of the fact that a subscriber line is subjected to all kinds of external interference, and the only protection against such interfering signals consists in providing the loop detection apparatus with a safety margin between the amplitude of the AC component after filtering and the detection threshold. This leads inevitably to an increased detection time, and the greater the desired immunity to external interference, the greater the detection time.
With an electronic junctor, and in particular with a junctor using a 12 volt DC generator, the safety margin has to be reduced to a minimum if performance is not to suffer.
Preferred embodiments of the present invention mitigate the difficulties of detecting a loop during ringing, particularly in the case of electronic junctors, and makes it possible for the loop to be detected with an adequate safety margin in the presence of external interference on the line.